At the present time the internal nature of material, such as the tissue of a human patient, may be examined by various means, including X-rays, ultrasonic echo reflections, and radioactive tracer techniques. Each of these techniques has serious limitations. For example, X-rays may be dangerous to unborn children and do not necessarily differentiate adequately between the various types of soft tissues of the body. Echo sounding using ultrasound, while it is sensitive to interfaces between soft tissues--that is what it does detect--cannot be numerically informative because one does not know the nature of the interface which causes the reflections. Ultrasonic reflections from interfaces do not give quantitative information on the nature of soft tissues. Radioactive tracer techniques do give some information on the nature of the metabolism of a tissue by whether the tissue takes up the radioactive tracer or not, but do not give information on the nature of the tissue itself.
In addition, no technique has yet been able to measure the temperature or the perfusion (blood supply) at an internal tissue location without inserting a mechanical device into the patient. Such temperature measurement may be important for certain types of heat treatment (hyperthermia) which have been used to treat some types of tumors (cancer).
The measurement of perfusion (blood supply to a tissue through the capillary network) may be useful for many medical purposes. While measurements have been made using invasive probes such as that of Bowen (Harvard Medical School, reported at 1981 "Summer School on Hyperthermia" at Hanover, New Hampshire, given by the American Association of Physicists in Medicine). No non-invasive method other than that of U.S. Pat. Nos. 3,771,355 and 4,059,010, discussed below, has been suggested.
It has been suggested that the velocity of sound in tissue may be used as a non-invasive way of determining the body temperature, see T. C. Cetas & W. G. Connor, Thermometry Considerations In Localized Hyperthermia, MEDICAL PHYSICS, Volume 5, page 79, 1978. However, sending a single sound beam through a patient and measuring the time of flight does not give quantitative information on a specific volume. In principle, it is possible to sweep the beam in an acoustic velocity computed axial tomographic fashion, called Acoustic Time-of-Flight Tomography. However, there are questions as to feasibility of such an approach.
For example, if all tissues examined with an Acoustic Time-of-Flight Tomograph had the dependence of ultrasonic velocity on temperature, such a technique would give an integrated temperature change over the line along which the sound beam is traveling, and not the temperature at any given point; and such a situation could be deciphered. However, since tissues do not all behave in the same fashion (the velocity in fat decreases with temperature while other tissues generally increase their velocity with temperature), it is now generally agreed that no useful information on temperature can be gained in this way.
In U.S. Pat. Nos. 3,771,355 and 4,059,010, both entitled "Ultrasonic Inspection and Diagnosis System" and incorporated by reference herein, an internal focal region ("focal point") of a substance, such as a soft tissue volume of a patient, is perturbed (heated) by ultrasonic waves from a perturbing field which may or may not be collimated or focused, depending on application. An ultrasonic sensing beam consisting of coherent bursts of oscillations is directed through the focal region. A comparison is made of the phase differences which occur in the sensing beam when it is directed through the region before and after the heating. That system, called "TAST" (thermoacoustic sensing technique) is difficult to apply to living organisms (substances) due to movement during testing. If the selected volume (focal region), which is typically a few millimeters across and 5-15 millimeters long, moves from one position before heating to a different position after heating, then the comparison will not be between exactly the same regions and will be inaccurate since it will be comparing different regions of tissue. Such variable inaccuracies constitute "noise" (interference) in the TAST system. Much more importantly, there may be a motion of the tissue which is not in the focal region but which is along the path of the sense beam. If there is motion of the tissue in the path of the sense beam in the TAST system, the time of flight can be changed and lead to large errors, even though this motion does not occur in the focal region. This phenomenon also constitutes system noise (interference).
The internal portions of a living organism, such as a human patient, are almost continually in motion. Some of the motion is rhythmic, for example, that due to the pumping of blood by the heart, or to respiration. Other movement may also be caused by voluntary and involuntary muscle movement (muscle artifact). Such motion, when it occurs along the path of the sense beam in the TAST system, may be a major cause of interference (noise) to the system.